The present invention relates to a retention device for crown caps, particularly used in a machine for forming the sealing gasket inside said caps.
Machines comprising two carousels which rotate about vertical axes are currently known for the formation of gaskets in crown caps.
A dose of plastic material in the pasty state is inserted in the caps at one of said carousels; said material is then compressed in the other carousel by means of appropriate punches so as to form gaskets which cover the bottom of the caps and ensure tightness when the caps are applied to containers.
So that the dose of plastic material first, and the gasket later, adhere to the inner surface of the caps, said caps are heated beforehand so as to liquify the internal paint which acts as adhesive between the gasket and the cap.
Heating occurs by conveying the caps through a magnetic field emitted by a high-frequency inductor. According to the type of material with which the caps are manufactured, a phenomenon of attraction toward the inductor occurs if the material is ferromagnetic, or a phenomenon of repulsion occurs if the material is diamagnetic, for example aluminum or stainless steel. In order to conveniently increase the efficiency of the inductors, the surface of the caps must pass very close to said inductors without however touching them, in order to avoid short-circuits or abrasions of the paint which covers them externally.
For the transfer of the caps from one carousel to the other, or for the insertion or extraction of the caps from the carousels, and also for the retention of the caps in the carousels, said caps rest toward the inside of appropriate accommodation seats which are defined on the periphery of rotating disks (so-called "stars") which are an integral part of the carousels, and rest, toward the outside, on fixed guides which extend concentrically to said disks.
A fixed cover is arranged above the caps and has the function of better guiding the caps during advancement, especially if said caps are made of a diamagnetic material which would cause their upward repulsion.
Known cap supporting systems have some severe disadvantages, which can be ascribed to the relative movements between the rotating disks and the stationary guides, which cause wear of the outer guides, caused by the friction of the caps as they advance, and of the seats of the stars, since the advancement of the caps causes the rotation of said caps in their seats. These wear phenomena are furthermore worsened by the magnetic attraction and repulsion force.
Another disadvantage which is observed in conventional cap conveyance systems is constituted by sparks due to the electric discharges which occur in the points of contact of the caps with the fixed guides and are caused by currents induced by the magnetic field. Said sparks lead to the wear of the guides and to damage of the caps.
The accommodation of the caps in the respective seats and the guiding thereof furthermore assume a certain play which, in view of the size tolerances of the caps, can cause a misalignment of said caps with respect to the punches and therefore centering errors in the formation of the gaskets.
A further not negligible disadvantage occurs during the passage of the caps from one carousel to the other, since in order to prevent the caps from being allowed to stray during this passage, deflectors are provided which constitute a constructive complication and in any case are troublesome when deformations or variations in the dimensions of the caps add to the negative effects of said plays.